what are the possible causes?
What do you mean by frothy? If it’s a milky sludge, this can be caused by short trips that do not warm up the engine enough to get rid of water condensation that builds up when the car is seating and a bad PCV valve. I had this problem with a 95 Dakota and a 98 Windstar. A longer drive once a week, an oil change and a new PCV valve fixed both. On the other hand, it could be a sign that coolant is getting into the oil via an intake or head gasket leak (feel free to correct me on this), this is a bad thing.
It would help if you provided more info such as make, model, year, mileage, driving habits, etc.
Agree with Ed; a little froth is normal, especially if you have been driving short trips without really warming up the engine. So, take a nice long freeeway cruise, and if you still find the froth, you have piston blowby or an internal leak somewhere letting coolant into the oil.
It can be a sign of a coolant leak coming from the intake manifold area.